Objective: The overall objective is to develop a field-emission x-ray source with high spatial coherence, and a phase-contrast imaging technique for clinical applications. Challenges: Current phase-contrast x-ray techniques generally require highly monochromatic, synchrotron-based, plane-wave radiation and sophisticated x-ray optics, so their clinical application is limited. Theoretical analysis and experimental studies have demonstrated the clinical feasibility of in-line phase-contrast imaging using a small, yet very bright x-ray source. However, these requirements cannot be met by the conventional sources. A novel field-emission x-ray source is therefore proposed. Methods: Cutting-edge nanotechnology will be applied to fabricate a super tip for the electron gun of the x-ray source. The super tip will be integrated with a miniature vacuum pump and an ion mirror to avoid contamination and the ion bombardment of the electron emitter. The anticipated end product is an x-ray source with a small focal spot (less than 0.025 mm), and at the same time, a high tube current (greater than 25mA), thus providing high spatial coherence. The field-emission x-ray source developed in the early phase of this project will be used to facilitate a new phase contrast imaging technique. The innovative system design is based on the in-line holography principle, acquiring phase contrast under large attenuation (a common clinical condition); therefore has the following clinically friendly features: (1) The source-to-detector distance of the system is no more than 1 meter; (2) the relative phase-contrast factor is up to 0.015 at 20 Ip/mm; (3) the detector quantum efficiency, radiation dose, and exposure time of the phase-contrast system are equivalent to that of the current state-of-art systems; and (4) the contrast-detail detectability is significantly improved due to the edge enhancement introduced by phase-contrast. Comprehensive measurements will be conducted to characterize the performance of the proposed system under clinical conditions. That includes objective measurements of resolution, contrast and quantum efficiency, and observer-based subjective measurements. The end product of the proposed research is a prototype phase-contrast imaging system for clinical applications. Summary: The proposed research, with its innovative field emission source and a new in-line x-ray imaging system, overcomes the major technical "roadblocks" in clinical applications of phase-contrast x-ray imaging.